Mark M. Green

A Helical Polymer with a Cooperative Response to Chiral Information

Polyisocyanates, long studied as theoretical models for wormlike chains in dilute solution and liquid crystals, differ from their biological helical analogs in the absence of a pre-determined helical sense. These polymers have an unusual sensitivity to chiral effects that arises from a structure in which alternating right- and left-handed long helical blocks are separated by infrequent and mobile helical reversals. Statistical thermodynamic methods yield an exact description of the polymer and the cooperative nature of its chiral properties. Minute energies that favor one of the helical senses drive easily measurable conformational changes, even though such energies may be extremely difficult to calculate from structural theory. In addition, the chiral nature of the polymer can be used to test theoretical ideas concerned with cholesteric liquid crystals, one of which solves the problem of assigning the helical sense.

The Macromolecular Route to Chiral Amplification

Cooperative phenomena, described by one-dimensional statistical physical methods, are observed between the enantiomeric characteristics of monomeric materials and the polymers they produce. The effect of minute energies associated with this amplified chirality, although currently not interpretable, can be easily measured. Nonlinear relationships between enantiomeric excess or enantiomeric content and polymer properties may offer the possibility of developing chiral catalysts and chiral chromatographic materials in which the burden of large enantiomeric excess or content may be considerably alleviated. New approaches to information and sensor technology may become possible.

The configurational stereochemistry of atactic vinyl homopolymers

Abstract Atactic vinyl homopolymers, synthesized from non-dissymmetric monomers, are produced racemic or enantiomerically pure depending on the length of the polymer chain. The optical inactivity of high molecular weight samples of these polymers arises by a mechanism unknown in small molecules: intermolecular compensation of diastereomers.

Trialkylphosphine oxide/amine stabilised silver nanocrystals—the importance of steric factors and Lewis basicity in capping agents

The preparation of specifically passivated silver quantum dots is reported. Investigations into the surface-capping agents have highlighted the importance of both the chain length and bonding strength of the individual ligands. Capping agents traditionally utilised to stabilise semiconductor quantum dots cannot always be used to effectively passivate metal structures, and a dual surfactant system was chosen. Long chain amines, usually weakly binding to a bulk silver surface, were found to be effective passivating agents. The steric properties of weakly bound trialkylphosphine oxides also appear to affect the particle stability. The choice of capping agents highlight the importance of both the Lewis base functionality and steric factors and these may play a key part in the design of future nanostructured materials. It also emphasizes that effective capping agents for nanodispersed metals are not necessarily the optimum ligands for other systems such as semiconductors, metal oxides, etc.

Fluorescence and Absorption Spectra of Biological Dyes

Data include charts and tables of absorbance and fluorescence of 4 coupling agents for immunity reactions, 34 typical fluorescent dyes and reagents, and 12 dyes or reagents having no fluorescence in solution but which may be fluorescent in the dry state. Since this report consists largely of reference data, the original must be consulted for details. Reprint source: The Perkin-Elmer Corporation, Norwalk, Connecticut, U. S. A.

Consequences of Cooperativity in Racemizing Supramolecular Systems

Saluting the sergeant: Phg-BTA (see scheme) cooperatively self-assembles into helical aggregates and shows unprecedented racemization behavior in the presence of base. In thermodynamically controlled conditions, the addition of a small amount of chiral auxiliary to this mixture results in a deracemization reaction and a final enantiomeric excess of 32 %. A theoretical model is presented to understand in detail the results obtained.

Homochirality in Life: Two Equal Runners, One Tripped

Strong arguments can be found in the literature addressed to the question of the origin of homochirality in life, supporting the hypothesis that primordial life could have evolved in both homochiral forms and that early on when life was still rarely found, random events led to the survival of only one of these living mirror images. This proposal is an alternative to the generally accepted view that small enantiomeric excesses of biologically important molecules were amplified to homochirality prior to life’s origin. Acceptance of the possibility of “two equal runners” leads to the importance of research investigations on routes to formation of ensembles of racemic mixtures of isotactic biologically interesting polymers, supramolecular entities and aggregates.

The synthesis of cadmium phosphide nanoparticles using cadmium diorganophosphide precursors

An investigation into the thermolysis of diorganophoshides in co-ordinating Lewis base solvents has revealed the importance of not only reaction conditions such as time and temperature, but also the importance of organic groups on the precursor. Nanoparticles of Cd 3 P 2 have been synthesised using compounds such as [MeCd(PBu t 2 )] 3 in tri-n-octylphosphine oxide (TOPO) and 4-ethylpyridine at elevated temperatures. Distinct optical characteristics of quantum confinement effects are observed as the particles dimensions decrease below the excitonic radius of 180 A.

Temperature-dependent stereoselectivity and hydrogen deuterium kinetic isotope effect for .gamma.-hydrogen transfer to 2-hexyloxy radical. The transition state for the Barton reaction.

The diastereomers of 5-deuterio-2-hexanol, prepared stereospecifically from acetol and lactic acid by using fermentation procedures, have been subjected to the Barton reaction with Ag/sub 2/CO/sub 3//Br/sub 2/ in pentane over the temperature range -8.65 to 30.05/sup 0/C. The derived 2,5-dimethyltetrahydrofuran deuterium incorporation yields both the C-5 diastereotopic hydrogen stereoselectivity and the isotope effect (k/sub H//k/sub D/) for the intramolecular hydrogen-transfer step. The isotope effect is classically temperature dependent, 5.82 (+- 0.07), 30.05; 6.01 (+- 0.22), 20.55; 6.81 (+- 0.17), 10.55; 6.90 (+- 0.08), 1.65; 7.47 (+- 0.49), -8.65/sup 0/C, while the stereoselectivity is small, 1.23 (1.65/sup 0/C), and temperature independent. Molecular mechanics calculations show that conformations with linear arrangements of C/sub 5/, H, and O show small steric energy differences between the diastereotropic hydrogens at C-5. Calculations of isotope effects and their temperature dependencies for model transition states match the experimental data for only C/sub 5/, H, and O angles greater than 150/sup 0/.

MASS SPECTROMETRY - A SENSITIVE PROBE OF MOLECULAR GEOMETRY

The unimolecular rearrangement chemistry of molecular ions encountered in mass spectrometers is ideally suited for stereoanalytical purposes. This follows from the spatial dependency of the bond making stage of the rearrangement. Examples are noted which demonstrate how such rearrangements may be utilized to assign configuration with and without deuterium labelling even when only one stereoisomer is available. These rearrangements may occur from molecular ions generated by electron impact or field ionization. Chemical ionization and ion cyclotron resonance mass spectrometry are known to be sensitive to stereoisomerism but have yet to be developed as stereoanalytical devices. Simple cleavage reactions of molecular ions may reveal stereoisomeric information when two diastereomers give rise to identical product ions. In the latter case appearance potential differences and product intensities differences mirror the enthalpic differences of the precursor neutral diastereomers. These thermochemical effects could be utilized to assign configuration to stereoisomers.